A pawl and ratchet assembly is a well known mechanical device. The ratchet comprises either a wheel, or linear, rack having a plurality of spaced-apart teeth. The pawl is pivotably mounted about a fixed point, adjacent the ratchet, such that the pawl is operable to rotate to engage one of the teeth of the ratchet, to prevent movement of the ratchet in at least one direction.
A ratchet and pawl assembly has numerous applications. One particular application is in a reel assembly, which may form part of a seat belt arrangement. A webbing strap is stored wrapped around a spindle pivotably mounted in a reel housing from which the webbing strap may be paid out. The initial amount of webbing strap paid out to secure the strap around a seat occupant depends on, inter alia, the size and position of the seat occupant. To allow the occupant freedom of movement and to carry out tasks, more webbing strap may be paid out temporarily during use, if the occupant moves from the standard seating position, for example when leaning forward. A torsional spring between the spindle and housing urges the spindle to rotate to retract the webbing, so as to take up any slack webbing when the occupant returns to the standard seating position.
When the occupant is subjected to an acceleration/deceleration outside of an acceptable range—for example in a crash situation—payout of the webbing strap must be prevented, so as to retain the occupant in the seat and thereby 2 reduce the risk of injury to the seat occupant. In a known reel, the spindle is associated with a ratchet wheel. A corresponding pawl is biased toward engagement with the ratchet and is associated with an acceleration sensor which, when triggered, releases and allows the pawl to engage with the teeth of the ratchet wheel, preventing further payout of the webbing strap and thereby retaining the occupant in the seat. It is important to stop the payout of the webbing strap as soon as possible once the acceleration sensor has been triggered.
In a crash situation, just before or as the pawl is triggered, the ratchet will begin to rotate (as a result of the inertia of the occupant in the seat causing the webbing strap to pay out). The pawl, being pivotably fixed relative to the housing, will rotate into a position where the tip of the pawl engages with a respective tooth of the ratchet wheel. The reactionary force of the pawl will prevent further rotation of the ratchet.
In order to effectively stop the ratchet, and prevent further payout of the webbing, there must be a sufficient amount of contact between the tip of the pawl and the tip of the teeth of the ratchet. Preferably, the tip of the pawl will be fully engaged with the entire leading face of a tooth of the ratchet at the point of contact. There is often a situation, however, where the pawl has not rotated towards the ratchet by a sufficient amount to allow full (or at least substantial) surface contact between the pawl and teeth by the time the pawl engages with the tooth.
In extreme situations of such marginal contact, the forces may be so great over the marginal contact area that the tip of either the pawl or the tooth of the ratchet shears off, or is at least deformed to some extent. Such damage would not only cause the ratchet to continue to rotate until the pawl engages with the next, undamaged, tooth but would also affect the reliability of the ratchet and pawl assembly in future operation. The assembly would need to be replaced.
Marginal contact between the pawl and tooth may cause the pawl to rebound from the ratchet altogether, allowing the ratchet to continue to rotate, and for webbing strap to pay out. The biasing of the pawl causes the pawl subsequently to re-approach the ratchet, for a second attempt at engagement. The contact between the pawl and tooth on the second attempt may be sufficient to avoid the pawl rebounding and/or the pawl/tooth being damaged. In extreme situations, this so-called “skip-lock” could continue for many iterations before there is a sufficient contact area between the pawl and tooth. This is undesirable.
To promote full engagement of a pawl with a ratchet tooth, the pawl may be pivotably mounted to a fixed point of the housing in such a way that, upon contact of the pawl tip with the teeth of a ratchet, the reactionary force imposed by the ratchet tooth on the pawl tip will act to create a torque (moment arm) about the pivot point of the pawl. Accordingly, in situations of marginal contact, the torque created in use may cause the pawl to fully engage with the entire surface of the tooth. However, whilst this known arrangement does conveniently promote full engagement of the pawl tip with the ratchet teeth, it still does not guarantee full engagement. The arrangement therefore does not reliably address the issues of tooth shear and skip-lock because it may still allow the pawl to take the full load of the reactionary force of the ratchet tooth before full engagement of the pawl and tooth has been established. The frictional forces caused by marginal pawl-tooth contact will be greater than the torque created by the offset pawl pivot point. Indeed it is known and has been proven in testing that the torque generated in this known arrangement is never able to provide sufficient driving force to overcome frictional forces due to extreme cases of marginal tooth-pawl contact and is therefore inherently and chronically susceptible to causing tooth shear damage in these situations.